1. Field of the Invention:
The present invention relates to the process of electrodialysis and to electrodialysis cells. More particularly, the invention relates to the use of electrodialysis to treat ultrafiltrate.
2. Brief Description of the Prior Art:
U.S. Pat. No. 3,663,405 discloses the treatment of an electrodeposition bath with ultrafiltration. In the ultrafiltration process, the desirable components of the electrodeposition bath, mainly resin and pigment, are concentrated on the ultrafiltration membrane surface from where they can be recycled to the electrodeposition bath for further use. The objectionable materials which accumulate in the electrodeposition bath, such as impurities introduced into the bath from the articles being coated, resinous decomposition products and excess solubilizing agents, pass through the membrane into the ultrafiltrate and thereby are continuously removed from the bath. A portion of the ultrafiltrate is purged to drain so as to remove the impurities from the system. The remainder of the ultrafiltrate is used for rinsing electrodeposited articles, and the rinse water which contains loosely adhering paint commonly called "drag-out" is returned to the electrodeposition bath.
U.S. Pat. No. 3,663,406 discloses the combined use of ultrafiltration and electrodialysis in treating an electrodeposition bath. In this patent, the electrodialysis is conducted within the confines of the bath. Although practicing electrodialysis within the bath provides excellent control of ionic impurities, it is very expensive and inconvenient. To be effective, it is necessary that one or more electrodialysis units be installed within the bath. If one unit is inactivated, there must be another unit operating, and the units are particularly susceptible to damage. For example, the article being coated often rips the membranes of the unit. Also, the membranes often become clogged with paint which inactivates the unit.
Japanese Patent Applications Nos. 77920/1978 and 124101/1978 disclose the combined use of ultrafiltration and electrodialysis to treat an electrodeposition bath but in which the electrodialysis cell is placed outside of the electrodeposition bath to treat the ultrafiltrate. Although using electrodialysis in this manner avoids some of the problems encountered when the electrodialysis cell is positioned inside of the bath, it creates other problems. It has been found that ionic materials in the ultrafiltrate can form a deposit on one of the electrodes impairing the electrical efficiency of the cell.